1. Field of the Invention
The present invention relates to a video data decoding method, more particularly, to a method for detecting and correcting an error in a video data received at decoder.
2. Description of the Related Arts
Error-robustness has been one of the major concerns in video communication, particularly in a system having a channel which is prone to many of errors such as a wireless communication system. Thus, much effort has been expended for enhancing the error-robustness of a transmitted data in a video communication system.
The methods for enhancing the error-robustness in video communication systems can be roughly divided into two categories in a viewpoint of a feedback channel concept. One is a feedback method and the other is a feedforward method.
In a typical feedback method, data is re-transmitted according to an interaction between a decoder and an encoder.
FIG. 1 is a conceptual block diagram of a wireless communication system in which the feedback method is applied.
When a decoder 15 fails to decode the received bit stream due to an error introduced in a channel, an error detector 16 detects the error or the portion of the data in which the error exists, and sends to encoder 11 feedback information such as a negative acknowledgment (NAK) for automatic-retransmission-request (ARQ) via either a control channel or a data channel. In addition to the retransmission of the data, the decoder 14 may ask the encoder 11 to perform INTRA update, with or without error tracking messages, or unequal error protection (UEP). In FIG. 1, the reference numerals 12 and 14 indicate RF units in the encoder and the decoder side, respectively.
Replenishment by these methods guarantees a fast error recovery to eventually enhance the picture quality.
However, the feedback channel method decreases the total channel throughput and increases the complexity of the communication system in which the method is implemented even though it offers fast updates of erroneous frames and promising quality of service (QOS).
Furthermore, one of the important issues in the feedback method is that the feedback information should not be corrupted by errors. More feedback information needs to be transmitted if the feedback information is corrupted, or the total system malfunctions until the system is refreshed by a correction.
On the other hand, the feedforward method involves just a slight increase in the complexity of a video encoder or a decoder for error-robustness and/or improves the quality-of-service (QOS) since it does not use a feedback information. In contrast to the feedback method, the feedforward method maintains the total channel throughput and the complexity of the communication system in which the method is implemented is quite Low.
There are two possible implementations of feedforward techniques: error estimation at an encoder and error detection/correction at a decoder.
FIG. 2A is a conceptual block diagram of a wireless communication system in which a feedforward technique of an error estimation type is applied.
In the system shown in FIG. 2A, an encoder 21 estimates channel errors by monitoring RF power or calculating error metrics by use of an error model 22 in order to refresh or unequally protect the encoded pictures. Then, the picture data is transmitted, along with estimated channel error information, to a decoder 26 via two RF units 23 and 25 and a channel therebetween.
Refreshment of certain parts of the possible erroneous picture produces fast error-recovery, while unequal error protection enhances the error-robustness. Provided that the error estimation model 22 in the encoder 21 is highly reliable, this method allows the decoder to have an improved quality of service (QOS).
However, this method usually adds auxiliary information for error-robustness, which may increase total bit budget. Two other disadvantages are vulnerability of the auxiliary information to channel errors during transmission and possible errors that occur in the unprotected region.
FIG. 2B is a conceptual block diagram of a wireless communication system in which a feedforward technique of an error detection/correction type is applied.
In the system shown in FIG. 2B, an error detector 36 gives a decoder 35 information on errors in the data transmitted from an encoder 31 via RF units 32 and 34 and a channel therebetween, so that the decoder 35 corrects the errors.
In this method, the error is detected and corrected only in the decoder side for incoming bit streams, as depicted in the FIG. 2B. One of the typical methods falling into the latter case is an error concealment method.
The basic idea behind this method is that, when the decoder locates erroneous syntax (e.g., administrative information, motion vectors, DC/AC coefficients, etc.), the decoder either replaces an erroneous group-of-block (GOB) or macro-block (MB) data with error-free GOB or MB data in the previous frame or utilizes motion vector information on GOB/MB in the previous frame for successful concealment of the current erroneous GOB/MB in order that subjective visual distortion is not noticeable.
Some of the advantages of this technique are relatively good visual quality at low bit rate and low complexity in implementation. However, at high bit rate communication where high picture quality is required, an error propagation effect can be noticeable due to possible accumulated errors.
Since we are mainly concerned about H.263 in the present invention, it is worth while mentioning some features of H.263 compared to H.261. One of the arresting features of H.263 is four negotiable options that definitely offers much improved performance over H.261. In addition, H.263 has much simpler syntax structure by eliminating a motion vector (MV) delimiter, which also provide better compression.
Meanwhile, the beauty of H.263 can be guaranteed only if the channel is highly reliable, e.g. in a wired ISDN and PSTN line in which the bit-error-rate has an order of magnitude of 10.sup.-8 or 10.sup.-9 such that it is negligible. However, advantages of H.263 cannot be guaranteed on an error-prone channel for the following reasons: (1) A wireless channel having both a random and a burst error undergoes corruption of information in a variable length coding (VLC) mode. (2) Negotiable options make administrative information, e.g., a macro-block type & coded block pattern for chrominance (MCBPC), have a full set of possible syntax elements for, which it is difficult to check errors at the decoder. In other words, added information due to four negotiable options also make H.263 bit stream vulnerable to channel errors in wireless links. (3) Due to the absence of a macro-block (ME) delimiter, the number of blocks to be discarded is increased.